1. Field of the Invention
The present invention relates to a phenol-formaldehyde composition. More particularly, the present invention relates to a composition comprising phenols and formaldehyde used for the preparation of phenolic resins, which is obtained by dissolving and absorbing formaldehyde into phenols.
2. Description of the Prior Art
Heretofore, compositions consisting essentially of phenols and formaldehyde have not been known, and, of course, uses for such compositions have never been considered.
As examples of conventionally known compositions containing formaldehyde, there are 37% formaldehyde(formalin), highly concentrated formalin, solid paraformaldehyde and the like.
Formalin contains 37wt.% formaldehyde, about 8-10wt.% methanol and the balance water. If the concentration of formaldehyde is low or the methanol content is high, the reaction rate with phenols is poor, whereby a long period of time is required for reaction. Further, about 63wt.% of water and methanol does not participate in the reaction and is necessarily later removed.
Highly concentrated formalin contains 43-47wt.% CH.sub.2 O, and the CH.sub.2 O concentration is larger as compared with the abovedescribed formalin. However, in order to prevent precipitation, methanol is further added so that the amount of methanol reaches 43-50wt.%. As a result, reaction with phenols is further slowed, thereby prolonging the reaction time and no advantages are obtained.
Paraformaldehyde is a solid material containing above 80wt.% of CH.sub.2 O and below 1wt.% of methanol, and, in order to obtain such, an aqueous formaldehyde solution must be subjected to a dehydration treatment. Since such a dehydration is conducted at high temperatures, molecular weight increases, whereby extended periods of time are required to mix and dissolve the same with phenols. On the other hand, however, since the reaction rate is high, the reaction system becomes heterogenous, whereby a gelled resin is apt to be obtained. Further, since paraformaldehyde is not in the form of a liquid, it is disadvantageous in charging it for reaction.
Further, with respect to the preparation of phenolic resins (e.g., resols or novolaks), phenols and a 37% aqueous solution of formaldehyde (formalin) have heretofore been mixed, heated and reacted in the presence of a catalyst, and the resins thus obtained have been dehydrated. However such processes involve the following disadvantages:
(1) Since the concentration of a phenol-formaldehyde reaction system is low, conversion is low, and, as a result, a large amount of unreacted materials remain. Therefore, the resins obtained are not sufficiently satisfactory in their properties, such as their heat resistance or curing rate.
(2) At dehydration, a large amount of water must be removed and production efficiency is poor. Further, the content of unreacted materials must be controlled in the dehydration step.
(3) In the case of transporting raw materials to the factory, phenols and formaldehyde must be separately and individually transported. Further, since formalin is formed in the form of an aqueous solution and then transported, reacted and dehydrated, such is very disadvantageous from the economical point of view.
On the other hand, in order to overcome the disadvantages of dehydration involved in conventional techniques, the direct reaction of phenols and formaldehyde polymers has been attempted, but, in such a case, the formaldehyde polymers do not easily dissolve in phenols, and in the case of trying to dissolve the same by heating, the thermal decomposition heat and the dissolution heat of formaldehyde and the reaction heat of phenols with formaldehyde are simultaneously generated, and, as a result, control of the reaction temperature is difficult, whereby the resins are gelated. Further, the properties of the resins obtained are poor in that gas generation at curing is large, heat resistance is poor, removal from a mold at molding cannot be smoothly conducted, etc.
Further, conventional resols are insoluble in water and are used in the form of varnish which is dissolved in organic solvents such as methanol, ethanol, benzene, toluene and xylene. However, organic solvents are generally inflammable and sometimes toxic. Thus, there are various disadvantages.
Heretofore, water-soluble resols have been prepared by reacting phenol and formalin in a highly alkaline environment (about pH 11) using caustic soda. However, due to residual catalysts in the resin, properties such as the insulation resistance after boiling in water are remarkably poor, and, as a result, it cannot be used as a resin for laminated boards and is merely used as an adhesive for plywood.
Further, in the case of preparing a resin for laminated plates, formalin is generally used within the pH range of 8 to 10, but, in such a case, a large amount of water is present in the reaction system, whereby the concentration of formaldehyde in the reaction system becomes low and the methylolation reaction is slow. Therefore, in order to increase the conversion of formaldehyde to above 65% (in case of it being below 65%, unreacted materials increase and yield is poor), a long reaction time is required, and since methylation proceeds during such a period of time, the resols obtained have a wide molecular weight distribution, i.e., have a large amount of methylolated polynuclear compounds. Therefore, resols having low affinity with water, i.e., low water-miscibility, are merely obtained and it is difficult to form water-soluble resols.
For this reason, in the case of using resols for laminated plates as a varnish, where they are diluted using only water to adjust the viscosity thereof, the varnish becomes turbid whereby uniform varnishes cannot be obtained. Therefore, it is unavoidable to use organic solvents such as methanol. Organic solvents are removed by evaporation in a drying step after an immersion step, but such is not preferred from the viewpoint of costs of venting treatment or the like. Further, there is the danger of explosion in the drying step.
Thus, if resols could be water-solubilized, it can be expected that varnishes in which water is used as a solvent could be obtained, it would not be necessary to use expensive organic solvents, there would be no danger of explosion and production capability could be increased by using elevated drying temperatures.
On the other hand, it has been considered to increase reaction rate using paraformaldehyde having a purity of above 80%. In such a case, since methylolation is very vigorous in the reaction in the presence of an alkali catalyst (pH above 8.0), it is impossible to produce stable resols because it is difficult to control the reaction temperature, side-reactions occur, or bumping or gelation is caused.